1. Field of the Invention
The invention provides dental devices, including orthodontic devices, associated restorative components and implants, of enhanced biocompatibility, corrosion resistance, and low modulus. More specifically, the invention provides dental devices, of all kinds, fabricated from a biocompatible, low modulus alloy containing predominantly titanium, niobium, and zirconium in specified amounts.
2. Description of the Related Art
Intraoral conditions provide a challenging environment for dental implants and other orthodontic and restorative devices, both from a chemical-biochemical and a mechanical-biochemical viewpoint. Nevertheless, standard dental implants and restorations are expected to last for the long term (greater than 10 years) in spite of severe conditions that include exposure to temperature ranges from about 5 to about 55.degree. C.; pH ranges from acid to base (3-10); multi-directional high forces (as high as about 2-24 N); stresses as high as 30,000 psi (207 MN/m.sup.2); and an average chewing cycle rate of about 80 per minute (i.e., in excess of about 10.sup.5 per year).
Most early dental restorations, predating 1950, failed clinically within a few years because of clinical, technical, or material-based conditions. The introduction of relatively inert cast cobalt alloys (Co-Cr-Mo), wrought titanium, titanium alloy (Ti-6Al-4V), aluminum oxide ceramic, carbon, and some polymers (primarily polymethyl methacrylate (PMMA)) increased longevity to an extent that they are now judged acceptable by clinical standards. Some of these devices have remained functional for decades.
It has been found that one-stage devices, those functionally loaded within 1-3 weeks after permanent placement into bone, demonstrate fibrous tissue growth adjacent to most biomaterial interfaces. These fibrous tissue zones are apparently caused by premature mechanical load-induced interfacial motion during bone healing. In contrast, under passive or non-functionally loaded conditions, most biomaterials demonstrate direct bone interfaces. When these devices are subsequently loaded for function, after a 3-6 month period of passive healing, interfaces evolve to remain as remodeled bone (osteointegration takes place) or subsequently change to soft tissues, depending upon the implant design, biomaterial, and host characteristics. Interface alteration from one tissue type to another (i.e., from bone to fibrous tissue), is dependent on both loading type (compression, tension, or shear) and magnitude.
Some investigators propose that fibrous tissue zones along dental implant biomaterial interfaces provide an important and favorable role related to force transfer. Therefore, fibrous tissue zones adjacent to dental implants have been called "pseudo periodontal ligaments."
Biomaterial-to-tissue interface conditions and attachment can be grouped as follows: (i) interfaces that demonstrate no attachment between biomaterial and bone, such as iron and cobalt alloys, polycrystalline aluminum and zirconium oxides, carbon and carbon silicon, and polymers; (ii) slightly attached interfaces, such as titanium and its alloys, alumina, and the like; and (iii) significantly attached interfaces, such as calcium-phosphate ceramics, bioglass, and glass-ceramics. It is suggested that interfacial attachment and stability are not solely dependent on the material surface but also require optimum implant design, materials, and clinical treatment.
There remains a significant need for new biomaterials to improve dental implants and devices and to provide for certain conditions that cannot be readily and effectively treated over the long term with available systems. Desirably, the biomaterials should have properties more similar to bone, be non-toxic and biocompatible, and implants of the biomaterials should have surfaces that allow development of a strong and stable bond to bone and soft tissues. Additionally, the materials should have excellent corrosion resistance coupled with high strength and toughness to withstand the rigors to which dental devices such as implants, abutments, attachment screws, orthodontic fixtures, wires, braces, bridges and the like, are subject in vivo. Many of these devices are subject to wide pH variations in the oral cavity and it is further desirable that they should be resistant to corrosion to avoid ion release into the body and also avoid producing a "metallic taste" in the mouth.